In the process of offset lithographic printing, a rotary cylinder is covered with a cylindrical surface referred to as a "printing plate" which has a positive image area that is receptive to oil-based inks and repellent to water as well as a background area that is repellent to the oil-based inks. The printing plate is rotated so that its surface contacts a second cylinder that is covered with a laminate having an ink-receptive rubber surface which is referred to as a "printing blanket". The ink present on the image surface of the printing plate transfers, or "offsets", to the surface of the printing blanket. Paper or other sheet stock to be printed is then passed between a nip formed by the blanket-covered cylinder and a rigid back-up cylinder or another blanket covered cylinder to transfer the image from the surface of the blanket to the paper. During the steps in which the image is transferred from the printing plate to the printing blanket and subsequently from the blanket to the paper, it is important to ensure intimate contact between the two contacting surfaces. This is ordinarily achieved by positioning the blanket-covered cylinder and the supporting cylinder or another blanket-covered cylinder that it contacts so that there is a fixed interference between the two. Therefore, the rubber-surfaced printing blanket laminate is generally compressed throughout the printing run to a fixed depth, typically about 0.002 to 0.006 inches.
If the printing blanket were constructed of solid rubber, it would bulge, or project radially away from the cylinder axis, in the areas adjacent to the nip when subjected to high nip pressure. This is because solid rubber cannot be reduced in volume and is therefore subject to lateral flow. Bulging would, of course, tend to distort the print image as well as possibly wrinkle the paper being printed. Therefore, compressible printing blankets have been developed.
To make the blanket compressible, a portion of the solid material making up the blanket is replaced by a gas, generally air. More specifically, layers beneath the surface of the blanket are constructed so as to contain millions of minute voids, which allow uniform compression to take place. As the voids beneath the area under pressure reduce in volume, they permit vertical compression--rather than lateral bulging--to take place at the cylinder nip. Conventional offset printing blankets generally include a multi-ply fabric base and a vulcanized elastomeric face. The threads of the fabric entrain a certain amount of air and provide voids and hence a certain amount of compressibility. To enhance the compressibility of such blankets, however, one or more cellular compressible layers is generally buried within or attached to one of the layers or fabrics between the base and the elastomeric face of the blanket.
Those skilled in the art have explored a wide variety of ways in which different open cell structures, closed cell structures, microspheres, and various combinations thereof can be used to obtain compressible layers that provide printing blankets having the desired compressibility properties. The numerous teachings of how to make compressible printing blankets include the teachings of Flint et al., U.S. Pat. No. 5,364,683; Larson U.S. Pat. No. 4,042,743; Shimura, U.S. Pat. No. 4,442,895; Rhodarmer et al., U.S. Pat. No. 3,795,568; Pinkston et al., U.S. Pat. No. 4,015,046; and Burns, U.S. Pat. No. 5,069,958.
In order to assure uniformity of printing, it is also important that compression be maintained uniformly over the entire length of the nip between the printing blanket and the support roll. Another important consideration relates to the handling of the paper or other webs being printed. Generally, cylindrical printing blankets used in various printing processes, are concaved on their outer surface to provide tension profiles across the width and between nips or contact points. Such tension profiles act to spread the web and prevent inward wrinkling.
FIG. 1 shows a structure representative of a concave-surfaced prior art cylindrical printing blanket. Printing blanket 1 is constructed around a metal support 2, typically configured in the form of a cylinder. A compressible layer 7 is thereafter formed by wrapping the coated support with a thread coated with an admixture comprising an elastomeric matrix 5 and a plurality of compressible cells 6. The wrapping conditions are controlled in such a manner that threads 4 sink to the lower portion of layer 7, adjacent to the coated support, whereas the remaining, i.e., upper, portion of layer 7 is comprised solely of the elastomer/cell admixture, substantially without any threads. After layer 7 is formed, it is partially dried or cured and a second fiber layer, i.e., of reinforcing fibers 8 coated with an elastomeric matrix 9 substantially free of cells 6, is applied upon compressible layer 7. Threads 4 and 8 act to reinforce the blanket in the same manner as layers of fabric would in non-cylindrical blankets. Matrix 9 acts as a subface for the printing surface 10. Subsequently, printing surface 10, for instance a solid elastomer such as a nitrile blend, is applied to the upper surface of blanket 1.
The degree of concavity of printing surface 9 as shown in FIG. 1 is exaggerated for illustrative purposes. However, it is manifest that a printing blanket or other roll surface that is concaved across its width varies in circumference around its cross-section, and that a point at either edge of the roll will travel further during a rotation of the roll than will a point at the center of the roll. Although this concavity solves the problem of paper wrinkling which is often encountered in the printing art, it leads, on the other hand, to the formation of unequal printing pressures and nip areas across the width of the blanket which, in turn, can cause undesirable results during printing, such as substantial dot gain and decreased print contrast values. In addition, it has a negative impact on the web feed tendencies.
Thus it would be desirable to have a printing blanket or similar compressible roll product that exhibits a uniform thickness across its entire width yet which enables the provision of tension profiles across the width and between nips or contact points in order to spread the web and prevent inward wrinkling.